The research is aimed at elucidating normal tissue responses to ionizing radiation. In particular, the response of gastrointestinal tract, skin, hair and testis will be investigated using techniques for in sito cell cloning. The response of lung vasculature will also be investigated by determining alterations in the tumor cell cloning efficiency in normal and irradiated lungs following innravenous injection of tumor cells. The response of spinal cord will be studied by determining the dose lethal to 50% of the animal following irradiation of a 6 mm segment of cervical spinal cord. The ultimate objective is to be able to reduce normal tissue injury relative to tumor injury in radiotherapy.